In an electrically powered steering apparatus that is provided as the steering system of an automobile, for example, a steering torque sensor commonly senses a steering torque applied to a steering shaft from a steering wheel by the steering operation of the driver. The steering torque sensor is normally configured from a magnetostrictive torque sensor. The steering shaft functions as a rotating shaft that rotates due to rotational force from the steering operation. The steering shaft constitutes a rotating shaft in the steering torque sensor. The electrically powered steering apparatus controls the driving of a steering force auxiliary motor according to a torque signal sensed from the steering torque sensor, and reduces the steering force for the driver to provide a pleasant steering feel.
As described above, magnetostrictive torque sensors are well known as steering torque sensors used in electrically powered steering apparatuses. In such a magnetostrictive torque sensor, magnetostrictive films that are magnetically anisotropic with respect to each other are formed at two specific locations on the surface of the steering shaft, for example. The magnetostrictive torque sensor has a configuration in which a non-contact system is used to sense changes in the magnetostrictive characteristics of the magnetostrictive films that correspond to the torsion of the steering shaft when torque is applied to the steering shaft from the steering wheel.
In the process of manufacturing a magnetostrictive torque sensor, a magnetostrictive film (in a wider sense, a magnetostrictive region) is formed over the entire circumferential surface in a specific surface in part of the steering shaft; i.e., over a specific axial width in the columnar rotating shaft; and then a process must be performed to provide this magnetostrictive film with magnetic anisotropy. Conventional methods for providing the magnetostrictive film with magnetic anisotropy in the manufacture of a magnetostrictive torque sensor involve applying a twisting torque to a rotating shaft on which a magnetostrictive plating (magnetostrictive film) is formed by an electrolytic plating process, for example, thus creating stress in the circumferential surface of the rotating shaft. This is followed by heat treating the rotating shaft in a thermostat while the shaft is kept under stress (see JP-2002-82000A, for example).
A conventional magnetostrictive torque sensor has, as the sensor configuration for sensing changes in the magnetostrictive characteristics of the magnetostrictive film without contact, a cylindrical sensor coil that encloses the magnetostrictive film around the periphery of the magnetostrictive film. The film is formed over the entire peripheral surface of the columnar rotating shaft. In common conventional magnetostrictive torque sensors, the width of the magnetostrictive film in the axial direction of the rotating shaft (hereinbelow referred to as “magnetostrictive film width”) has tended to substantially match the length or width of the sensor coil in the same axial direction (hereinbelow referred to as “sensor coil width”). The magnetostrictive film width and sensor coil width, and the distance between the magnetostrictive film and the inner peripheral surface of the sensor coil (hereinafter referred to as the “gap”) are defined as dimensions relevant to the relationship between the sizes and placements of the magnetostrictive film and the sensor coil in the magnetostrictive torque sensor.
The following is a description of the problems relating to the magnetostrictive film width, the sensor coil width, and the gap in a magnetostrictive torque sensor, made with reference to FIGS. 9 and 10.
FIG. 9 shows the relationship between the positional misalignment (horizontal axis: mm) in the axial direction of the rotating shaft between the magnetostrictive film and the sensor coil, and the rate of change in the sensitivity of a conventional magnetostrictive torque sensor the sensor (vertical axis). In this case, the gap is 0.5 mm. FIG. 10 shows the relationship between the gap (space in the radial direction of the rotating shaft, horizontal axis: mm) between the magnetostrictive film and the sensor coil, and the rate of change in the sensitivity of the same magnetostrictive torque sensor (vertical axis).
Using a case with no positional misalignment (0.0 on the horizontal axis, and a rate of change in sensitivity of “1” on the vertical axis) as a reference in the graph 101 in FIG. 9, the rate of change in sensitivity decreases below “1” if positional misalignment occurs either upward (to the right in FIG. 9) or down-ward (to the left in FIG. 9). For example, the rate of change in sensitivity exhibits the characteristic of decreasing below 0.98 when positional misalignment in the axial direction of the rotating shaft exceeds±0.68 mm, for example.
A magnetostrictive torque sensor is a device that requires high sensory precision. Therefore, if the rate of change in sensitivity falls below 0.98, the driver may experience an unpleasant sensation in regard to steering responsiveness in cases in which the sensor is actually installed in an automobile or the like.
According to the graph 102 in FIG. 10, the rate of change in sensitivity is 1.0 in the same manner when the gap on the horizontal axis is 0.5 mm. Based on this value, when the gap is doubled to 1 mm; i.e., when the gap is misaligned by 0.5 mm, for example, the rate of change in sensitivity exhibits the undesirable characteristic of decreasing to about 0.85.
In a magnetostrictive torque sensor, the sensitivity properties of the sensor decrease when positional misalignment occurs in the positional relationship between the magnetostrictive film formed on the rotating shaft and the sensor coil placed around the periphery of the magnetostrictive film. Particularly, when such a magnetostrictive torque sensor is used in an electrically powered steering apparatus of an automobile, it is undesirable for such positional misalignment to occur because the electrically powered steering apparatus creates an unpleasant sensation when operated.
In the environment in which a current electrical power steering apparatus is manufactured, the expected positional misalignment during the assembly process is about 0.2 mm when the sensor coil is attached, and about 0.2 mm when the steering shaft (rotating shaft) is attached. Therefore, there is a possibility that the sensor coil and the steering shaft will be misaligned in position by a maximum of about 0.4 relative to each other.
An electrically powered steering apparatus incorporated into an automobile may undergo a maximum positional misalignment of about 1 mm, including the positional misalignment during manufacturing, due to changes over time or excessive input from the road surface, depending on the environment in which the apparatus is used. Therefore, there is a need for structural resistance to be provided to magnetostrictive torque sensors used in the electrically powered steering apparatuses of automobiles, such that positional misalignment is less than 1 mm even at a maximum, including misalignment that occurs during manufacture and subsequent operation.
Therefore, a need exists for developing a magnetostrictive torque sensor, and an electrically powered steering apparatus that uses this sensor, wherein the dimensional relationship between the shapes of a magnetostrictive film and sensor coil formed on the rotating shaft can be optimized to improve the sensitivity characteristics of the sensor, and the positional misalignment tolerances during assembly of the apparatus can be increased.